The present invention relates to a case for an instrument of small volume, in particular a watch, including a middle part and a removable back cover applied under the middle part.
This type of case is commonly used to cover a wristwatch. The back cover may either be screwed on or snap fitted and may have a sealing gasket interposed between the back cover and the middle part. A back cover of this type can only be set in place or removed using special tools and these operations are generally reserved for a watchmaker, as is apparent in the two designs taken as an example hereinbelow.
Swiss Patent No. 251 447 shows a back cover snap fitted under a middle part. The case includes a middle part-bezel, a casing frame freely fitted in the middle part, a crystal the heel of which is clamped between the inner edge of the middle part-bezel and the casing frame, and a back cover. The back cover is fitted freely in the middle part and is pressed against a compressible sealing gasket lining the bottom face of the casing frame, by two levers each pivoting on an edge located on the inside each of the two opposite sides carrying the lugs of the middle part.
Swiss Patent No. 329 625 shows a back cover applied under a middle part by means of a screw system. This watch includes a crystal of which the edge bent towards the exterior is pressed against a gasket by a casing ring on which it rests. In order to hold the latter in the case, and so that it exerts a pressure against the edge, the back cover, whose contour is rectangular like the middle part, is fitted into the bottom opening of the latter and has, on its outer face, an annular housing, of which the outer wall is tangential to the sides of the opening and open at these tangent locations. In this housing there is arranged an externally threaded round ring, which is screwed into the middle part at the aforementioned tangent locations. Notches facilitate the screwing and unscrewing thereof.
In the design types briefly described above as in all the other designs known to date, if the watch is fitted with a battery in order to operate, the user will have no other choice than to give the watch to a person skilled in the art to change the battery once the battery reaches the end of its life.
It is true that in order to overcome this drawback, certain case back covers are fitted with a battery hatch which consists of a cap screwed on or fixed by a bayonet fitting in the back cover, the cap being provided with a screw slot which allows the user to open the hatch himself by means of a coin, as is implemented for example in the Swatch watch (registered trademark).
However, this solution would not be suitable for a battery, or other power source, with a large diameter as is found in pager watches or in watches fitted with a GPS system. It would also not be suitable for a particular watch including at least two batteries. For this type of instrument of small volume, it is thus necessary to find a solution which allows the entire back cover to be removed in order to have free access to the battery or batteries which have to be changed, such access having to remain simple to implement by the user, and without having to use any tools.
It will also be noted that in this type of instrument (pager or GPS for example) the power source has to be changed frequently, due to the high power consumption of the system. The battery change must thus be able to be effected quickly and by the user himself who will keep spare batteries on him.
In order to overcome this problem, the present invention proposes a back cover held under the middle part by means of at least one rail, this rail having a first longitudinal portion arranged to be slid into a groove made in the middle part and a second longitudinal portion arranged to cap an edge of the back cover when said first portion is introduced into said groove to fix said back cover under said middle part.